1. Field of the Invention
The present invention relates to an induction heating apparatus, and more particularly to an induction heating apparatus suited for a continuous induction heating apparatus of metallic materials in a sheet or foil form.
2. Description of the Prior Art
As is well known, an induction heating apparatus comprises an inductor or a heating coil provided adjacent to a conductive material. An alternating magnetic field is induced in the heating coil, such that an eddy current I is caused to flow through the conductive material by virtue of an electromagnetic induction effect. The conductive material then gives rise to the so called Joule heat I.sup.2 R due to the inherent electrical resistance R of the said conductive material.
Hitherto an induction heating apparatus has been utilized in, for example, annealing a strip metallic sheet material. Application of an induction heating system to the heating of such a strip metallic sheet material may be classified as a longitudinal flux heating method as shown in FIG. 1(A) and a transverse flux heating method as shown in FIG. 1(B). However, the longitudinal flux heating method can not be applied to a non-magnetic material such as aluminum or the like, although the same can be applied to a magnetic material. In order to solve such a shortcoming, a transverse flux heating method as shown in FIG. 1(B) was developed, which was invented see U.S. Pat. No. 2,448,010, Baker, et al., Aug. 31, 1948, entitled Transverse Flux Induction Heating Furnace Structure. The principle of the method is well known without a further explanation.
FIG. 2(A) shows a sectional view of an apparatus for induction heating a strip metallic sheet material 3 such as aluminum in accordance with a transverse flux heating method and also shows the directions of a secondary current i2 generated in the strip metallic sheet material. The strip metallic sheet material 3 is transferred in the direction of the arrow a between opposing cores 1 and 1' of the magnetic material "E" letter shaped in section. The magnetic flux .phi. is generated in the arrow direction as shown, whereby a magnetic circuit is formed. The secondary current i2 or an eddy current as shown in FIG. 2(B) is generated in the strip metallic sheet material 3 as a function of the magnetic flux .phi. generated as shown in FIG. 2(A). The strip metallic sheet material 3 is heated by virtue of the secondary current i2. The strip metallic sheet material 3 is heated, while the material 3 is transferred in the direction of the arrow a. Accordingly, the strip metallic sheet material 3 is rather uniformly heated in the longitudinal direction throughout its major portion by virtue of the secondary current flowing in the width direction, although the edge portions 3a of the strip metallic sheet material 3 are additionally heated by virtue of the secondary current flowing in the longitudinal direction. As a result, the edge portions 3a are excessively heated, and uniform heating throughout the width direction becomes difficult. This will be described in more detail with reference to FIG. 3(A). FIG. 3(A) shows the portion 4 in FIG. 2(B) in an enlarged manner, wherein a hatched portion shows a portion which is influenced correspondingly by the core and to that end the same is identified by the same reference character 1 for simplicity. FIG. 3(A) shows a density distribution of the secondary current i2. The flow path of the secondary current i2 is determined by the core 1 and the current i2 flows from the portion A to the portion B. In such a case the density of the secondary current per a unit time is higher at the edge portion C, since the current flow component in the longitudinal direction is inevitably generated, with the result that the portion C is overheated. This is also caused by the fact that the heat transferrence at the edge is as small as a half of the central portion.
However, even in such a conventional method, an approximate uniform heating pattern could be obtained, if the cores 1 are disposed such that the end of the core is positioned slightly inside of the edges of strip metallic sheet material 3 as shown in FIG. 3(B). Nevertheless, in this case, a requirement must be met that the distance d as shown in FIG. 3(B) should be constant. Accordingly, a tiresome problem is encountered that each time the width of the strip metallic sheet material 3 is changed the core suited therefor must be replaced.
Thus, according to induction heating of the strip metallic sheet material by a transverse magnetic flux heating method, two major shortcomings were involved, namely, that uniform heating throughout the width direction of the strip metallic sheet material is difficult and that uniform heating of various strip metallic sheet materials having a variety of width and thickness by the use of the same induction heating coil is difficult.